CaIif.-30 



SOME RELATIONS OF TEMPERATURE TO GROWTH 

AND INFECTION IN THE CITRUS SCAB 

FUNGUS CLADOSPORIUM CITRI 



BY 

H. S. FAWCETT 



Reprinted from JOURNAL OF AGRICULTURAL RESEARCH 

Vol. XXI, No. 4 : : : : : Washington, D. C, May 16, 1921 




PUBLISHED BY AUTHORITY OF THE SECRETARY OF AGRICULTURE, WITH 
THE COOPERATION OF THE ASSOCIATION OF LAND-GRANT COLLEGES 



WASHINGTON : GOVERNMENT PRINTING OFFICE : 1921 



D i 



qfrV 



SOME RELATIONS OF TEMPERATURE TO GROWTH 
AND INFECTION IN THE CITRUS SCAB FUNGUS 
CLADOSPORIUM CITRI » 

By H. S. Fawcett 

Professor of Plant Pathology, Citrus Experiment Station, College of Agriculture, 

University of California 

INTRODUCTION 

In some previous work by the author (j) 2 it was found that the results 
of different inoculation tests on rapidly growing sour-orange trees {Citrus 
aurantiumh. {13) with Cladosporium citri Massee were variable. It had 
previously been observed in the citrus orchards of Florida that abundant 
infections from scab did not inevitably follow the presence of abundant 
moisture on rapidly growing tissue, although these two conditions were 
usually present when abundant infection did occur. Seasons were 
encountered when scarcely any infection from scab occurred, even though 
conditions of moisture and growth appeared to be ideal for an outbreak. 
The author was led to suspect that temperature was also an important 
factor in infection. 

The experiments which form the basis of this paper were planned to 
determine what influence different temperatures might have, first, upon 
infection by the fungus when the two other conditions previously men- 
tioned, abundant moisture on the leaves and rapid growth were main- 
tained, and second, on the growth and spore formation of the causal 
fungus on culture media. These experiments with the scab fungus 
carried on during intervals in a more extended temperature investigation 
with other citrus fungi reported elsewhere (4) are seen to be somewhat 
incomplete, but they appear to throw considerable light on the possible 
relation of temperature to the occurrence of citrus scab and to offer a 
more complete explanation for the differences in the occurrence of scab 
from year to year or from one season to another. It is, of course, realized 
that in the orchard, under natural conditions, temperature is fluctuating 
and not constantly maintained as in these experiments. Nevertheless, 
the experiments indicate at least the comparatively narrow range of 
temperature within which infection of a very susceptible host is possible 
under the presumably ideal conditions. 

1 Paper No. 72, University of California, Graduate School of Tropical Agriculture and Citrus Experiment 
Station, Riverside, Calif. 

2 Reference is made by number (italic) to "Literature cited," p. 253. 

Journal of Agricultural Research, (243) Vo '- XXI, No. 4 

Washington, D. C. May 16, 1921 

xv Key No. Calif .-30 



244 Journal of Agricultural Research vol. xxi.no. 4 

Citrus scab has been described and illustrated in considerable detail 
in previous publications by Scribner (8, 9), Swingle and Webber (14), 
Fawcett (2), and others. Its main characteristics are corky wart-like 
projections on leaves, fruit, and small twigs, due to the attack of the 
fungus on tender, rapidly growing tissue. The portion of the leaves 
not attacked usually maintains a healthy green color close up to the edge 
of these warts or scabs. 

Considerable confusion has resulted, however, in the identity of the 
causal organism, Cladosporium citri (1,2). It should be mentioned that 
the fungus is an unusual Caldosporium and very different from the 
ordinary type of Cladosporium such as Cladosporium herbarum Lk., for 
example. This causal relation of Cladosporium citri to scab was at 
one time questioned by Grossenbacher (5), probably because he was 
working with another Cladosporium as indicated by the characteristics 
mentioned in his paper. The author's identification of Clodosporium 
citri as the causal organism has been confirmed by Hesler (6), Stevens (12), 
and Home (3), as well as by his own later work (3). Moreover, Cladospo- 
rium citri has been found by Stevens (10) to be the cause of a similar 
disease known as avocado scab in Florida. Sour-orange (Citrus aurantium) 
(13) and lemon (Citrus limonia Osbeck) trees appear to be the most sus- 
ceptible hosts to citrus scab. Satsuma orange (Citrus nobilis var. unshiu 
Swingle), trifoliata orange (Poncirus trifoliata Raf.), and pomelo 
(Citrus grandis Osbeck) are also attacked. Sweet orange (Citrus sinensis 
Osbeck) is nearly immune. In most of the infection experiments here 
reported, the sour orange was employed because it was one of the most 
susceptible species. 

The experiments were made in 191 7 at the Laboratory of Plant Physi- 
ology, Johns Hopkins University, in surroundings entirely free from the 
disease in question. The various temperatures employed were main- 
tained for the most part by means of the apparatus described by Living- 
ston and Fawcett (7). 1 

TEMPERATURES AT WHICH YOUNG CITRUS LEAVES WERE INFECTED 

A preliminary test on sour-orange seedlings was made on May 16, 191 7, 
with 8-months-old plants, which had been potted about three weeks and 
were beginning to develop two or three new leaves each. These plants 
were placed for three days in the different temperatures indicated below 
and were inoculated by drawing over the surface of the leaves and shoots 
a camel's-hair brush moistened with spores and bits of mycelium of the 
fungus in distilled water. 

The average temperatures of the nine different chambers were 12 , 16 , 
19 , 23 , 26.5 , 31 , 34.5 , 38. 5 , and 42. 5 C, with a fluctuation of i° 
to 2 . One plant was exposed in each chamber for a period of three 
days in a saturated atmosphere and then taken out and the surface water 

1 The author wishes to acknowledge the aid rendered by Dr. B. E. Livingston in these experiments. 



CHft 

Author 

1W : m 



May 16, 1921 Temperature Relations of Cladosporium citri 245 

allowed to dry. The plants that had been inoculated at 16 , 1 9 , and 23 
subsequently became diseased. The plant from the 16 chamber had 
scabs just visible 7 days from date of inoculation. Those from the 19 
and 23 ° chambers were not seen to be diseased until 10 days later. The 
plants that had been inoculated at 16 and 19 had many scabs. The 
plant that had been inoculated at 23 was only slightly diseased even on 
the seventeenth day after inoculation. 

Controls which were not inoculated, but on which distilled water was 
drawn over the leaves, were subjected to 16 and 26.5 ° C. and kept under 
the same conditions as the others, but no scab developed upon them. 

A second test on sour-orange seedlings just like the first was made on 
June 5, 1 91 7, except that different temperatures were maintained in 
some of the chambers and two plants were left in each temperature, and 
they remained for 54 hours instead of 3 days. Control plants not inocu- 
lated were used at 20 and 27.5 ° C. The average temperatures in this 
test were 14 , 20 , 24.5 , 27.5 , 32.5 , 36 , 40 , and 44.5 . 

In this test one of the plants inoculated at 20 C. developed scabs on its 
leaves. These were first noted two weeks after inoculation. None of the 
other plants developed scabs. As will be seen in Table I, the chamber at 
20 is the only one that lies in the range at which infection occurred in the 
other two tests, namely, 16 to 23 . The chambers at 14 and 24.5 , 
therefore, appeared to be outside, one below and the other above, the 
range for infection. 

On September 24, 191 7, a third inoculation test was made with sour- 
orange trees in pots, about one year from seed, on which small new leaves 
were developing. The plants were surrounded by cylinders of blotting 
paper kept moist by absorption of water from below. Another piece of 
blotting paper was placed over the top of the cylinder. The different 
temperature chambers (7) in which the plants were left for 3 days 
were maintained nearly constant within a fluctuation of about 0.5 C. 
At the end of this 3-day period when they were taken out, the moist 
cylinders of blotting paper were removed, and the plants were set in the 
greenhouse. In about % hour the moisture had evaporated from the 
surfaces of the plants. At the same time that these were taken out, two 
other plants were inoculated with the fungus as before and were set in the 
same greenhouse and under the same conditions as the others. These 
were not surrounded by blotting paper but were allowed to dry like the 
others, after water with spores had been drawn over their leaves. 

The temperatures at which the plants remained during the 3-day 
period in the saturated atmosphere of the dark chambers were 13.5 , 16 . 
18.5 , 21 , 24.5 , 27.5 , and 32 C, respectively. 

The temperatures recorded in the greenhouse for the 12 days after the 
plants were taken out of the temperature chambers gave an average 
minimum of 15 C., and an average maximum of 30 . 



246 Journal of Agricultural Research voi.xxi,No.4 

Only those subjected to 16 , 18.5 , and 21 C. for the 3 days after 
inoculation developed any scab on their leaves after removal to the 
greenhouse benches. 

On plants inoculated at 16 C. scab was just visible in 6 days from the 
time of inoculation, or 3 days from the time of removal from the temper- 
ature chamber. In 2 weeks the scab lesions were well marked. 

Plants inoculated at 18. 5 C. showed a slight indication of scab 5 days 
after inoculation, or 2 days after plants were removed from the temper- 
ature chamber. In this case the scab lesions were well marked in 1 1 days 
from time of inoculation. 

Plants inoculated at 21 C. showed a slight indication of scab 5 days 
after inoculation, as was true of those at 18.5 . In 7 days the diseased 
spots were quite distinct, and in 1 1 days they had developed into definite 
and typical scabs. On plants inoculated at 13. 5 , 24.5 , 27.5 , and 32 , 
however, no sign of lesions developed. 

Control plants were also kept under the same conditions, but none 
of these developed scab lesions at any time. The two plants which were 
inoculated with the fungus at the time the others were taken out of the 
temperature chambers and from whose surfaces the water was allowed at 
once to evaporate also remained free from scab. These were considered 
to be additional controls. The nonappearance of scab on these two plants 
indicated that infection at exposures of 16 , 18. 5 , and 21 C. developed 
because the hyphae had penetrated while the plants were still in the 
temperature chambers. This conclusion is also supported by the fact 
that the time from inoculation to the appearance of visible scabs agrees 
well with that observed in previously reported tests (2, 3). The time 
elapsing between the removal of the plants to the greenhouse bench and 
the first appearance of scab appears generally to be too short to represent 
the incubation period of the fungus within the tissues of the host. 

On September 28, 191 7, a set of young pomelo seedlings with leaves 
just begining to unfold was inoculated in the same manner as that 
described for the sour-orange seedlings of the third test, except that these 
were left four days in the temperature chambers instead of three. With 
the exception of one leaf on a plant inoculated at 18.5 C, which developed 
a slight indication of disease but no distinct scab, there was no visible 
sign of disease on any of the plants up to October 17, when the last 
observation was recorded. 

In all four tests just considered, scabs were always confined to rapidly 
growing leaves. The older leaves always remained free from the disease 
in every test. Neither was there any development of scabs on any of the 
large number of similar plants kept in the greenhouse and not inoculated. 



May 16, 1921 Temperature Relations of Cladosporium citri 



247 



Table I. — Range of infection as indicated by the four inoculation tests on citrus 


seedlings l 




Results of inoculation at temperature (°C.) of — 


inoculation. 


12 


13- S 


14 


16 


18.5 


19 


20 


31 


'i 


74-5 


26.5 


3J-S 


31 


32 


3*-S 


34- S 


36 


38. S 


40 


42- S 


44-S 


Sour orange: 

(1) May 16, 1917. . . 









* 




* 


* 


* 
O 


* 











O 
O 

O 









O 





O 





O 









O 
O 


O 





(3) Sept. 24, 1917. . 
Pomelo: 

(4) Sept. 28, 1917- ■ 









* 



* 
* 



































1 * Indicates definite infection; o, no infection. 

INOCULATION OF DETACHED LEAVES 

Young sour-orange leaves were detached and placed in Petri dishes, 
one set containing distilled water on which the leaves were floated and 
the other set containing cornmeal agar. Some of the fungus mycelium 
was placed in contact with the leaves, and the preparations were allowed 
to remain in the temperature chambers for 15 days. The temperatures 
used were 13.5 , 16 , 18.5 , 21 , 24.5 , 27.5 , and 32 C, with a fluc- 
tuation of about 0.5 . Infection took place in water at 16 , 18.5 , 
21 , 24.5 , and 27. 5 , but not at 13. 5 nor at 32 ; and in cornmeal agar 
at 18.5 and 21 , but not at 13. 5 , 16 , 24.5 , 27.5 , nor 32 . 

INFLUENCE OF TEMPERATURE ON GROWTH AND SPORE PRODUC- 
TION OF CULTURES 

In addition to the inoculation experiments described above, the rate 
of growth and the formation of spores were studied with different main- 
tained temperatures. Distilled water, in which was floating a young 
sour-orange leaf, and cornmeal agar were used as culture media. 
Inoculation was accomplished by means of a disk (2.5 mm. in diameter) 
of agar medium bearing the mycelial weft. The cultures were kept in 
the dark, maintained temperature chambers for three or four days. 
At the end of the period the radial extension of the hyphae from the 
transferred disk, was determined. Observations were also made on the 
general abundance of spores. The data obtained are brought together 
in Table II. The growth rate is seen to have been very slow as com- 
pared with that of many other citus fungi (4). For the time employed 
and for the temperatures used the greatest extension of hyphae occurred 
at 21 C. This growth rate was smaller, for each temperature, in corn- 
meal agar than in distilled water with the leaf, except at the two higher 
temperatures. At 32 ° no extension of hyphae was seen in water, while 
in cornmeal agar the enlargement was about one-sixth of that obtained 
at 21 for the same period. 

Spores were observed on the marginal hyphae in from 24 to 48 hours in 
all cases where growth was observed, except in the agar culture at 32 C. 
The first examination for presence of spores was made after 48 hours in the 
agar cultures. Spores were abundant at the first examination upon the 
growing hyphae of all the test cultures at 21 °, one of the temperatures 



248 



Journal of Agricultural Research 



Vol. XXI, No. 4 



at which infection of rapidly growing sour-orange seedlings and detached 
sour-orange leaves had been observed to be pronounced. Spores were 
also abundant in the water preparations at 24.5 and 27. 5 after 24 hours, 
and in the preparations of agar at 24.5 after 48 hours; at these tempera- 
tures no infection had occurred in the tests with growing plants, but a 
slight infection had been observed after 1 o days on detached leaves in water. 

Spores that had fallen from the hyphae in certain ones of these cultures 
were found to have fallen on the surface of the medium at some distance 
laterally from the ends of the aerial hyphae on which they had been 
borne. Apparently these spores had been ejected with considerable 
force from the ends of the hyphae. In one case at 1 6° C. a fringe of spores 
was observed, most of them at least 210 microns distant from the ends of 
the outermost hyphae. This feature of spore dispersal in cultures had 
previously been seen but not recorded. It was noted as most frequent 
after four days in cultures growing at the edges of leaves in cornmeal 
agar at 16 , 18 , 21.5 , and 24.5 . 

In the 1 6° preparation with water and a sour-orange leaf, spores were 
observed to be mostly formed on the tip ends of the outwardly extending 
aerial hyphae at some distance from the floating leaf. They were nearly 
hyaline at first, subsequently becoming slightly dusky. Most of them 
were 1 -celled, but a few were 2-celled. Detached spores were germi- 
nating from the ends of spores in line with the longest axis. Hyphae were 
hyaline when viewed singly, but were pinkish or flesh-colored in mass. 
On cornmeal agar under the same conditions spores were forming with 
from two to four in a chain, and the ejected spores were germinating 
on the surface of the medium. 

The influence of temperature on the vegetative hyphae was marked. 
At 27. 5 and 32 ° C. in the cornmeal agar preparations the hyphae were 
broad and straight, 8 to 12 microns or more in diameter. At 23 and 21 , 
on the other hand, the hyphal diameter was only one-half as great and 
the hyphae were more bent and tortuous. At 18.5 , 16 , and 13.5°, 
the hyphae were broader, much as at the higher temperatures. 

Table II. — Growth and spore formation in Petri-dish cultures of Cladosporium citri at 
different maintained temperatures 





Fungus floating in water with tender leaf. 


In cornmeal agar. 


Temperature. 


Radial 

growth in 

3 days. 


Spore formation. 


Radial 

growth in 

4 days. 


Spore formation. 




In 1 day. 


In 3 days. 


In 2 days. 


In 4 days. 


°C. 

13-5 
16 

18. s 

21 

24- 5 
27-5 
3 2 


Microns. 
62 
170 
285 
655 
385 




Few 

...do 

...do 

Abundant . . 

...do 

...do 

None 


Few 

Abundant.. 

Few 

Abundant 1 . 
Abundant.. 
Abundant 1 . 


Microns. 

38 

92 

116 

37° 

3 2 3 

197 

62 


Few 

...do 

...do 

Abundant 1 . 
...do 

Few 

None 


Few. 

Do. 

Do. 
Abundant. 

Do. 
Few. 
None. 



1 Spores found some distance laterally from ends of the outermost hyphae as if ejected with considera- 
ble force. 



May 16, 1921 Temperature Relations of Cladosporium citri 



249 



Table III. — Number of days from inoculation to first indication of disease at different 

temperatures 1 





Days after inoculation at temperature (°C.) of — 




16 


18.5 


19 


20 
14 


21 

5 

4 
3 


23 


24- s 


26. s 


27- 5 


Sour-orange seedlings: 

(1) May 16, 1917 


7 
6 

6 


5 

5 

4 


17 


17 


10 


— 




(2) Tune ■;. 1017 








Detached leaves: 

(5) In water 


IO 







1 A dash indicates that no scabs developed on plant placed at the respective temperatures. A blank 
space indicates that no plants were used in a given test at these respective temperatures. 

DISCUSSION 

Infection of sour-orange seedlings was limited to a range of about 8° C, 
16 to 23°, inclusive, in these experiments. This range could not have 
been over io° in extent under the same conditions even if more tempera- 
tures had been investigated, since plants inoculated at 14 and at 24. 5 ° 
failed to become diseased. This range is seen to include the temperature 
(21 ) at which the fungus was found to have the most rapid growth 
in water and in cornmeal agar medium. The temperatures just outside 
the range on the lower side (14 and below) are those at which the fungus 
grew somewhat more slowly than it did at the temperatures just outside 
the range on the upper side (24.5 and above). If, however, we consider 
the experiment with detached leaves in water (Table III) in which an 
exposure of 15 days was given at the different temperatures, the range 
for infection, though not extended downward, was extended upward to 
27. 5 , a temperature at which the hyphal growth in water was approxi- 
mately the same as that at the lower limit of the range, 16 (Table II). 
This may indicate that the rate of extension of the hyphae must be above 
a certain minimum before infection of the host can occur, a point that 
may have some significance. The rate of extension of the hyphae also 
appears to have a relation to the time between inoculation and first 
appearance of scab as is shown, in a general way, in Table III. This 
time period, with a single exception, is shortest with temperature at or 
near 21°, the temperature for maximum growth if the tests on different 
dates be considered separately. 

It is, of course, not to be definitely concluded that infection can not 
occur with temperatures outside the range within which it was con- 
fined in these experiments, but it appears probable that scab is not to 
be expected with temperatures that remain outside this range. Of 
course, orchard temperatures may vary so as to subject the trees to 
temperatures within the range of infection for a sufficient time period to 
allow the penetration of the fungus. No attempt was made to determine 



250 Journal of Agricultural Research vol. xxi, No. 4 

the minimum time necessary for the fungus to become established in 
the host tissue at the different temperatures listed in order to produce 
scab subsequently. 

The interrelation of the host and parasite also demands consideration. 
Temperature, of course, influences the host as well as the parasite, and 
the lack of infection in these experiments at certain temperatures may 
have been due to special physiological conditions or states either in the 
host or in the parasite, or even in both. It may be suggested that there 
was not enough time for infection in those cases where it failed to occur. 
From the experiments in which leaves floating in water were left for a 
long time in contact with spores of the fungus at different maintained 
temperatures, it appears that, even with long periods of time, there are 
certain temperatures, not fatal to either parasite or host, at which in- 
fection does not take place. At some temperatures the fungus even 
grew fairly well on the surface of the leaf without producing infections 
leading to scab. 

The relation of temperature to infection by the scab fungus, as here 
brought out, seems to suggest a satisfactory explanation for some differ- 
ences in the results of inoculations previously recorded by the author (2). 
A set of inoculations made in August, 1906, failed to produce scab, 
while a similar set made in January, 1909, was successful. An exam- 
ination of the weather records for the locality where the experiments 
were made shows that the mean daily temperatures for the two weeks 
following the August experiment fluctuated between a minimum of 23 
and a maximum of 30 C, while the corresponding temperature range 
for the January experiment lay between a minimum of 1 1 ° and a maxi- 
mum of 1 9 . In the former case, the daily means were above the range 
for infection of seedlings in the experiments here reported except at its 
minimum, 23 , while the other case shows an overlapping of the range 
of daily means with the range for infection, to 19 . Since the tempera- 
tures in the greenhouse where the 1909 experiments were performed 
were probably higher than those outside, the successful infections in 
January were almost certainly due to the fact that the temperatures 
fell for the most part entirely within the range that is favorable to 
infection. 

The temperature relation here emphasized is probably important in 
determining the occurrence of scab in the orchard and in the explana- 
tion of scab epidemics. But temperature is only one of a number of 
conditions, such as stage of growth, humidity, time period, etc., that 
must be fulfilled for scab to occur. 

Among the various conditions discussed by Grossenbacher (5) as favor- 
ing the development of scab, the following general observations seem to 
bear on this temperature relation. 

If the air is fairly dry and the weather mostly warm and bright during the develop- 
ment of the first spring flush, scab may fail to develop. On the other hand, the disease 



May 16, 1921 Temperature Relations of Cladosporium citri 



251 



often becomes very severe in groves if the weather is cold and wet during the develop- 
ment of the first spring growth. 

Again: 

Some trees in scabby groves under observation during the past two years have 
retained the late starting habit and remained practically free from the disease. 

Again : 

During the early spring of 1915 the air was very moist and cold while the first growth 
was in its early stages and as a result sour scab developed in great abundance even 
on high sandy land if growth was early and vigorous. 

Stevens (10) says of citrus scab : 

If cool wet weather prevails at the time the new growth is putting out or at the time 
the fruit has set, the disease is apt to be severe ... in groves where scab has become 
established. 

In the light of these temperature experiments, the greater severity 
of scab at the low spring temperatures of Florida seems to be due largely 
to the fact that these temperatures fall mainly within the range for 
infection, while the later spring and summer temperatures are usually 
too high for infection to take place, even though other conditions are 
favorable. An examination of the temperature data for Tampa, Fla., 
given in Table IV suggests that this last statement may be translated 
into terms of mean temperatures, to the effect that severe infection will 
usually occur when the mean temperatures are well within the infection 
range and that infection will be unlikely to take place when the mean 
temperatures are outside this range. 

Table IV. — Mean temperatures at Tampa, Fla. 
[In degrees centigrade] 



Month. 


1909 


1910 


1911 


1912 


1913 


1914 


191s 


1916 


1917 


1918 


February 

March 


16.5 
i9- S 
23 


i5-5 
19 
20. 5 


17-5 
20 

23 


13-5 
20. 5 

23 


18. s 

21 

21 


15-5 

16. s 
22 


*5-5 
14-5 
20. 5 


16.5 
17 
20. 5 


16.5 
20. 5 
22 


20 


April 








Mean .... 


i9-5 


18.5 


20 


*9 


20 


18 


17 


18 


20 


21 


May 


24. 5 

27 

26.5 


24-5 
26. 5 
26.5 


24-5 

27 

27 


25- S 

26 

28 


24 
26 

27 


25 
27 
27 


26 

28 
28.5 


25 
26 
28 


24 

26.5 

28 


24-5 

27 
28 




Tulv 






Mean 


26 


26 


26 


26.5 


25-5 


26.5 


27-5 


26. 5 


26 


26.5 



The average of the monthly mean temperatures for February, March, 
and April for this io-year period was between 17 and 21 ° C, while the 
average of the monthly mean temperatures for May, June, and July was 
between 25. 5 and 26. 5 . All the monthly mean temperatures for the 
first period were well within the infection range for scab, while all the 



252 Journal of Agricultural Research vol. xxi, No. 4 

monthly means for the second period are well above the infection range 
as determined by the inoculation experiments. 

It is of interest to note also that the lowest average for February, 
March, and April was 17 C. in 1915, the year mentioned by Grossen- 
bacher (5) as one in which scab developed in great abundance on the 
growth starting in early spring. These data taken in connection with 
the maintained temperature experiments seem to explain the frequent 
absence of scab on tender growth of the second or third cycles even in 
the moist periods of summer. The spores may be present, the moisture 
and growth conditions may be favorable, but the temperature may be 
too high. 

The occurrence of severe late scab in some seasons is probably due to 
a decided drop in the temperature, bringing it well within the infection 
range for a sufficient length of time for infection. 

SUMMARY 

(1) Experiments to determine the influence of temperature on scab 
infection on young sour-orange (Citrus aurantium) leaves and on the 
growth and spore formation of the causal organism (Cladosporium citri) 
are reported. 

(2) The inoculation temperatures resulting in infections of growing 
plants under conditions of rapid growth and abundant moisture were 
1 6°, 1 8. 5 , 1 9 , 20 , 21 , and 23 C. No infections were obtained under 
the same conditions on plants inoculated at 12 , 13.5 , 14 , 24.5 , 26. 5 , 
27-5°, 3i°, 32°, 32-5°, 34-5°, 36°, 38.5 , 40 , 42.5 , and 44.5 . 

(3) Detached leaves floated in water with the scab fungus were infected 
at i6°, 18.5 , 21 , 24.5 , and 27.5 C. 

(4) The temperature at which the greatest extension of hyphae of 
the causal organism in cultures was observed was 21 ° C. The highest 
temperature at which extension was observed in water was 27. 5 , and 
in cornmeal agar 32 . 

(5) Spores were observed in 48 hours or sooner in all the temperatures 
at which growth took place except 32 ° C. At certain temperatures the 
spores appeared to be ejected with considerable force from the ends of 
the hyphae. 

(6) The temperature at which the time was shortest between inocu- 
lation and first observance of signs of disease was usually 21 ° C. in the 
different tests. This time increased toward the upper and lower limits 
of the infection range. 

(7) This limited range of temperature at which infection of a sus- 
ceptible host took place under the presumably favorable conditions of 
the experiment appears to explain the great differences observed in 
the occurrence and severity of scab from year to year and from season 
to season in citrus orchards. It also explains the differences in results 
of previous inoculation experiments not hitherto understood. 



Mayi6, 192 1 Temperature Relations of Cladosporium citri 253 

(8) The conditions necessary for scab infection indicated by these 
experiments are (1) viable spores of the fungus, (2) young citrus leaves 
of a susceptible species, (3) moisture, and (4) temperatures between 16 
and 23 C. 

LITERATURE CITED 
1) Fawcett, H. S. 

1910. CLADOSPORIUM CITRI MASS. AND C. ELEGANS PENZ. CONFUSED. In MycO- 

logia, v. 2, no. 5, p. 245-246. 



1912. citrus scab. Fla. Agr. Exp. Sta. Bui. 109, p. 5i-[6o], fig. 24-31. 
References, p. [60]. 



1916. citrus scab. In Phytopathology, v. 6, no. 6, p. 442-445. 



THE TEMPERATURE RELATIONS OF GROWTH IN CERTAIN PARASITIC FUNGI. 

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